The present invention relates generally to an improvement of a nuclear reactor in which a moderator and a coolant are isolated from each other by using a calandria tank.
More particularly, it relates to a nuclear reactor having a structure in which a calandria tank containing a moderator is surrounded by a pressure container containing a coolant to thereby isolate the coolant from the moderator, the coolant in the pressure container is recirculated by a recirculation pump attached to the pressure container, and steam and water are separated in the pressure container.
A nuclear reactor in which a moderator and a coolant are separated from each other can be represented by a heavy-water-moderated boiling-light water-cooled pressure tube type reactor which uses heavy water as a moderator and light water as a coolant.
The conventional pressure tube type nuclear reactor of this type has a nuclear reactor structure in which, as shown in FIG. 3, a number of pressure tubes 14 penetrate through a calandria tank 12 that contains heavy water 10, each of the pressure tubes 14 contains therein a fuel assembly 16, and the calandria tank 12 is placed in an iron/water radiation shielding 18.
Around the nuclear reactor are arranged inlet tubes 20 for supplying a coolant into pressure tubes 14, outlet tubes 25 for guiding the coolant (two-phase fluid consisting of steam and water) boiled in the pressure tubes 14 into steam drums 22 in which steam is separated from water, recirculation pumps 24 for recirculating the separated water, and downcomer 26 for guiding the separated water from the steam drums 22 to lower headers 28.
Light water of a lower temperature and low pressure is allowed to flow through a gap between the calandria tank 12 and the iron/water radiation shielding 18 to effect the cooling. The coolant (light water) at a high temperature and high pressure that flows through the pressure tubes 14 is in a separate system. By means of the recirculation pump 24 in this system, the coolant recirculates through a path consisting of lower header 28.fwdarw.inlet tube 20.fwdarw.pressure tube 14.fwdarw.outlet tube 25.fwdarw.steam drum 22.fwdarw.downcomer 26.fwdarw.recirculation pump 24.fwdarw.lower header 28. Further, the heavy water 10 which serves as a moderator is separate from the above two systems; i.e., the heavy water at a low temperature and low pressure flows through the calandria tank 12 to effect the cooling.
The nuclear reactor of the above-described pressure tube type has advantages in that it is a heavy water reactor exhibiting good neutron economy, that plutonium utilization characteristics are excellent, and that the fuel assembly can be exchanged during the operation of the reactor.
As described above, however, it is required to provide the iron/water radiation shielding and its cooling system, many groups of inlet tubes and outlet tubes, steam drums and headers, thereby causing the structure to become complex. Therefore, the construction of the plant requires extended periods of time.